1. Field of the Invention
The present invention relates to a highly efficient feedforward amplifier and a control method thereof.
2. Description of the Related Art
In order to provide coverage areas to keep up with the rapid spread of mobile communications in these years, many base station equipment need to be installed. Some base station equipment need to be installed in the same location of the existing base station for installation reasons. On the other hand, there is a demand for reduction of power consumption of base station equipment. In these circumstances, there is a growing need for miniaturization and lower power consumption of base station equipment. Base station equipment typically includes devices such as modulator and demodulator, transmitter amplifier, heatsinks, and various controllers. Since the part in the base station equipment that consumes the majority of the power is the transmitter amplifier, there has been a focus of attention in regard to reduction of power consumption of the transmitter amplifier.
The transmitter amplifier in the base station equipment uses a linearization technique because of (1) the need for simultaneous amplification of multiple carriers and (2) the need for meeting standard values such as an adjacent channel leakage power required in a mobile communication standard. A feedforward amplifier is known as an amplifier employing linearization technique. FIG. 1 shows a basic configuration of a feedforward amplifier. The feedforward amplifier 100 includes a signal cancellation circuit 10 and a distortion eliminating circuit 20 (Non-patent literature 1).
Non-patent literature 1: N. Pothecary, Feedforward linear power amplifiers, Artech House, 1999.
The signal cancellation circuit 10 includes a divider 11 which distributes a signal input in the feedforward amplifier 100 into two paths, a vector adjuster 12, a main amplifier 13, a delay line 14, and a combiner/divider 15. A path including the vector adjuster 12 and the main amplifier 13 is a main amplifier path PMA and a path including the delay line 14 is a linear transfer path PLT. The combiner/divider 15 is typically implemented by a directional coupler and has a degree of coupling equivalent to the gain of the main amplifier path PMA.
The main amplifier 13 amplifies an output signal from the vector adjuster 12. The combiner/divider 15 combines an output signal (here, a signal consisting of a main wave component which is an input signal of the feedforward amplifier and a distortion component generated by the main amplifier) from the main amplifier path PMA and an output signal from the linear transfer path PLT. The vector adjuster 12 adjusts the amplitude and phase of an input signal of the main amplifier 13 so that the amount of the distortion component output to a distortion injection path PDI described below becomes sufficiently large (the adjustment is referred to as loop adjustment of the signal cancellation circuit 10). A signal extracting unit and a controller required for loop adjustment of the signal cancellation circuit 10 are not shown. The distortion component is an input signal of the distortion injection path PDI. The combiner/divider 15 outputs the main wave component and the distortion component to the other path of the distortion eliminating circuit 20 (the main amplifier output transfer path PMT, which will be described later).
The distortion eliminating circuit 20 includes a delay line 21, a vector adjuster 22, an auxiliary amplifier 23, and a power combiner 24. A path including the delay line 21 is a main amplifier output transfer path PMT and a path including the vector adjuster 22 and the auxiliary amplifier 23 is a distortion injection path PDI. The vector adjuster 22 adjusts the amplitude and phase of the distortion component input in the distortion injection path PDI so that the adjacent channel leakage power ratio (ACLR) of an output signal of the power combiner 24, which will be described later, becomes sufficiently small (the adjustment is referred to as loop adjustment of the distortion eliminating circuit 20). A signal extracting unit and a controller required for the loop adjustment of the distortion eliminating circuit 20 are not shown. The auxiliary amplifier 23 amplifies an output signal of the vector adjuster 22. The power combiner 24 combines an output signal of the main amplifier output transfer path PMT and an output signal of the distortion injection path PDI with equal amplitudes, opposite phases, and equal delays. As a result, the distortion component is eliminated and the main wave component is output from the feedforward amplifier 100.
In this way, the signal cancellation circuit 10 detects the distortion component generated by the main amplifier 13 and the distortion eliminating circuit 20 injects the detected distortion component into the output signal of the main amplifier 13, with equal amplitudes, opposite phases, and equal delays. By this operation, the feedforward amplifier 100 compensates for the distortion component generated by the main amplifier 13.
If there is no other active circuit in the rest of the feedforward amplifier 100, power consumption of the feedforward amplifier 100 is determined by the power consumption of the main amplifier 13 and the auxiliary amplifier 23 which are active circuits. The power efficiency of the feedforward amplifier is the ratio between the output power and power consumption of the feedforward amplifier.
A method for increasing the power efficiency of the feedforward amplifier 100 is to reduce the power consumption of the active circuits in the feedforward amplifier 100 while maintaining linearity. However, reduction of power consumptions of the main amplifier 13 and the auxiliary amplifier 23 reduces a current supplied to each amplifying element and therefore increases distortion components generated by the amplifying elements. There is a trade-off between reduction of power consumption and distortion generated.
If the power consumption of the auxiliary amplifier 23 is reduced, the distortion component detected by the signal cancellation circuit 10 is further distorted in the auxiliary amplifier 23 and consequently a distortion component that differs from the distortion component to be eliminated is generated. As a result, the distortion component generated by the main amplifier 13 cannot sufficiently be eliminated. The auxiliary amplifier 23 has to linearly amplify the distortion component detected by the signal cancellation circuit 10. Therefore, usually a Class A amplifier is used as the auxiliary amplifier 23 and its power consumption cannot significantly be reduced.
Main amplifiers to which a high-efficiency amplification technique is applied have been proposed in order to improve main amplifier power efficiency. One of such main amplifiers is a Doherty amplifier (Patent literature 1). The Doherty amplifier includes a carrier amplifier and a peak amplifier (Non-patent literature 2). When the input power of the Doherty amplifier exceeds a certain value, the peak amplifier operates and an output from the peak amplifier is combined with an output from the carrier amplifier. The Doherty amplifier can achieve high power efficiency because the carrier amplifier is operating in saturation in an input power region in which the peak amplifier operates. It has been reported that the power efficiency of a 2-GHz-band feedforward amplifier for W-CDMA can be improved by 2% with the Doherty amplifier used as its main amplifier (Non-patent literature 3).
Patent literature 1: U.S. Pat. No. 6,320,464
Non-patent literature 2: S. C. Cripps, Advanced Techniques in RF Power Amplifier Design, Artech House, 2002.
Non-patent literature 3: K-J. Cho, J-H, Kim, and S. P. Stapleton, “A highly efficient Doherty feedforward linear power amplifier for W-CDMA base-station applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 1, January 2005.
The nonlinear characteristic of the Doherty amplifier is generated on different principles in a region in which the peak amplifier operates and a region in which the peak amplifier does not operate. In the region in which the peak amplifier does not operate, the nonlinear characteristic of the Doherty amplifier is that of the carrier amplifier. In the region in which the peak amplifier operates, the nonlinear characteristic of the Doherty amplifier is the combination of that of the carrier amplifier and the peak amplifier. The Doherty amplifier is capable of achieving high power efficiency in the region in which the peak amplifier operates. However, the nonlinear characteristic of the Doherty amplifier is complicated compared with that of the carrier amplifier alone.
A feedforward amplifier that uses the Doherty amplifier as its main amplifier should compensate for the complicated nonlinear characteristic of the Doherty amplifier. If distortion of the feedforward amplifier is ideally compensated for, all distortion components contained in an output signal of the Doherty amplifier are eliminated. However, actual feedforward amplifiers cannot completely eliminate distortion components generated by the main amplifier. This is because adjustments for achieving equal amplitudes, opposite phases, and equal delays in the signal cancellation circuit and the distortion eliminating circuit have limitations and because the frequency characteristics of the signal cancellation circuit and distortion eliminating circuit do not completely compensate for the frequency characteristic of the complicated nonlinearity generated by the Doherty amplifier. Therefore, there is a problem that while a high power efficiency can be achieved in a situation in which the peak amplifier operates, distortion cannot sufficiently be compensated for due to the complicated nonlinear characteristic. If the Doherty amplifier is used as the main amplifier in order that the ACLR may be less than or equal to a specification value specified in a radio communications standard, the power efficiency cannot be improved because an output back-off of 5 dB or so is required.